Tube with highly porous target



eQw. GOETZE ETAL TUBE HITH HIGHLY POROUS TARGET 2 Sheets-Sheet 1 Filed Dec. 3, 1962 0 A H a M lllf l lllLlR INVENTORS Gerhard W. Goeize and Alvin H. Boerio.

WITNESSES Mg R. S

BY f g ATTORNEY Oct. 19, 1965 (5. w. GOETZE ETAL 3,213,316

TUBE WITH HIGHLY POROUS TARGET Filed Dec. 5, 1962 2 Sheets-Sheet 2 20o I I T I E I (9 I I I I0 I FAST RESPONSE I SLOW RESPONSE I l O 2.5KV/CM ELECTRIC FIELD zooI I50- I TOTAL GAIN FIg. 5. 5 I00- 3 SECONDARY EMISSION WITHIN THE LAYER GAIN SECONDARY EMISSION FROM THE LAYER GAIN fir VT VE i F i .6. o g

United States Patent 3,213,316 TUBE WITH HIGHLY POROUS TARGET Gerhard W. Goetze, Monroeville, and Alvin H. Boerio,

Turtle Creek, Pa., assiguors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Dec. 3, 1962, Ser. No. 241,641 12 Claims. (Cl. 315-12) This invention relates to an image storage system and more particularly to those in which information is stored on a target member and the information read out by means of an electron beam.

One well known type of storage target electrode pickup tube is the image orthicon. The image orthicon has a target structure formed of a thin film of glass. A photocathode is positioned on one side of the glass target and a focusing means is provided for directing the photoelectrons emitted from the photocathode onto the write side of the target. The photocathode emits electrons in response to a light scene directed thereon and a charge pattern is established on the target corresponding to the light scene. An electron gun is positioned on the other side of the target and provides an electron beam of low velocity to scan the read side, opposite to the write side, of the glass target to deposit electrons on each elemental area of the target in proportion to the charge pattern established on the write side. The remaining electrons in the scanning or reading electron beam are returned to a collector electrode to provide the output signal of the image orthicon.

In the image orthicon pickup tube, the photo-electrons, in striking the target, initiate secondary emission from the bombarded surface or writing surface and these secondary electrons are collected by a mesh adjacent the bombarded surface. This collector mesh is held at a positive potential with respect to the cathode of the scanning or reading electron beam. The reading action is accomplished in the image orthicon by the landing of the low velocity electrons from the reading beam onto the read side of the target. If the lateral target resistivity is made high enough to permit reasonable storage times, the transverse resistivity will accordingly be high and more electrons may leave the writing side of the taiget than reach it through the target. In this way, the write side may charge to an equilibrium potential, a potential slightly more positive than the collector mesh, so that as many electrons leave the target as land on it. No further effective transfer of pattern information can take place under these circumstances and the tube loses its capability for writing. The basic problem therefore associated with the image orthicon target is that of providing transverse charge neutralization within a frame time, while at the same time providing a target of sufficient resistivity to limit lateral leakage of charge to obtain adequate storage.

Another type of well known pickup tube is that utilizing a target exhibiting the property of electron bombardment induced conductivity. This type of tube also provides high sensitivity and is similar to the image orthicon in sensitivity at low light levels. In the electron bombardment induced conductivity type of target, there is provided a thin metallic conductive coating or back plate of a material such as aluminum which is spaced from a photocathode input. This back plate is electron permeable and is coated with a film of a high resistivity material such as a semiconductor or insulator material on the opposite side of the metallic film with respect to the photocathode. The high resistive material utilized in the structure is of the type which exhibits the property of solid state conduction when bombarded with high energy electrons. This property has been termed EBIC. A

3,213,316 Patented Oct. 19, 1965 ice typical type of material utilized in such a device is arsenic trisulfide. A low velocity scanning or reading electron beam may be used with a velocity below first crossover to maintain the exposed surface of the insulating coating at a fixed or equilibrium potential different from a potential established on the back plate. The electrons emitted from the photocathode are accelerated by suitable means, penetrate the target and induce electron bombardment induced conductivity through the target, that is, between the surface scanned by the reading electron beam and the back plate. This high velocity bombardment by the writing beam modifies the potential on the exposed surface from the equilibrium value toward the potential of the back plate and a charge pattern is established corresponding to the light pattern focused on the photocathode. The output signal may be derived from the target in a similar manner as that described with respect to the image orthicon in which the low velocity scan beam is utilized and the output signal is derived from the electrons returned to a collector electrode. It is also possible to derive a signal from the conductive back plate due to simple capacitive coupling when the charge is restored on the scan of the target member. This is similar to the Well known vidicon type pickup tube read operation.

In the EBIC target, the primary electrons increase conductivity of the target as a result of creating a number of secondary electron-hole pairs. This increase in conductivity may provide gains of 1,000 or greater. In order to obtain high gains within the target, the EBIC material should have a low band gap, high value of carrier mobility and low transit time. In addition, the film should be thin. However, in order to avoid excessive dark current it is necessary to limit the number of carriers present in the unexcited film. This requires that the resistivity of the material be at least 10 ohm-centimeters found only in materials requiring band gaps of 2.0 electron volts or greater. Loss of signal voltage strength on the target and lag effects limits the use of excessively thin films. The signal voltage strength is inversely proportional to target thickness. In addition, the large values of capacitance associated with very thin EBIC targets are responsible for serious persistence effects, which show up in terms of excessive time being required for writing information on the target and reading it off.

In addition, the requirement that the material have a band gap of 2.0 electron volts or greater requires the use of materials having low mobility charge carriers of about 10 cm. /volt sec. This also results in serious persistence effects. The time for reading oif and erasing a charge image on an EBIC target may require several seconds. This makes the EBIC type of target unsuitable in many applications where fast response is desired.

In a US. Patent 3,189,781, issued June 15, 1965, entitled Electron Discharge Device by Lempert, and assigned to the same assignee as this application, another type of target is described. The target described therein uses a transmissive type of electron multiplier. This structure also exhibits inherent target gain as does the electron bombardment induced conductivity type of target. The writing beam generates secondary electrons which are emitted from the opposite or scan surface of the storage layer which is the surface which is scanned by the reading beam. It was found that this target providedadequate lag characteristics in some embodiments but the target is lacking in sensitivity.

It is accordingly an object of this invention to provide an improved storage system which utilizes a target structure' which provides high inherent amplification within the target itself.

It is another object to provide an improved storage system of fast response time.

It is another object to provide an improved storage target system that is free from serious persistence effects.

It is another object to provide an improved storage device to provide a stable target member which may be processed without danger of damage to the material.

It is another object to provide an improved storage target system which permits low electric fields to be used across a storage target during operation and prevent dark current background, defects and blemishes in the image.

It is another object to provide an improved storage system capable of storing and integrating information for long periods of time.

It is another object to provide an improved storage target relying substantially on conduction of electrons within a vacuum.

Briefly, the objects of this invention are accomplished by providing a storage system utilizing a porous storage layer of high resistivity material provided on a conductive back plate as the storage target. The input radiations are directed onto the porous or spongy insulator or semiconductor and generate free electrons of low energy within the porous storage layer, a first portion of which escapes from the exposed or exit surface of the storage layer to thereby charge the exposed or exit surface in a positive direction. A collector mesh or electrode is provided adjacent the exposed surface of the porous storage layer to collect the secondary electrons emitted therefrom. In addition, a relatively low electric field is impressed across the porous storage material by means of an electron scanning beam or reading beam which maintains the exposed or exit surface of the storage material at an equilibrium potential and a conductive back plate which is maintained at a fixed potential positive with respect to the equilibrium potential. In this manner, free electrons which are generated by the input radiations within the porous storage material which do not escape as secondary electrons are collected by the conductive back plate. This collection of electrons by the back plate also charges the exposed surface of the storage material in a positive direction to enhance the effect of the secondary emission of electrons from the exit surface. By providing an extremely porous storage material (less than percent of normal bulk density) and a low electric field across the porous storage layer, one is able to obtain multiplication within the target by electrons emitted from the exit surface and collected by the collector and by electrons conducted through the vacuum spaces of the porous layer to the conductive back plate, without resort to solid state conduction through the bulk material. This results in a target operation of high gain, fast response and very good storage and integration characteristics combined with freedom from dark current structure in the image.

Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this description.

For a better understanding of the invention, reference may be had to the accompanying drawings in which:

FIGURE 1 is an elevational view in section, schematically representing a pickup tube and associated system in accordance with the teachings of this invention;

FIG. 2 is an enlarged elevational view in section illustrating the electrode target assembly in FIG. 1;

FIG. 3 is an elevational view in section of a modified electrode target assembly with associated voltages that may be embodied in the tube shown in FIG. 1;

FIG. 4 is a graphical representation of the gain of the target as a function of the electric field across the storage layer;

FIG. 5 is a graphical representation of the gain of the target as a function of the exit surface potential for a constant input signal; and

FIG. 6 is a graphical representation of the integrated signal as a function of the exit surface potential.

Referring in detail to FIG. 1, there is illustrated a pick up tube incorporating the teachings of our invention. The tube comprises an envelope 10. A face plate 12 is provided in the envelope 10 and is transmissive to the desired scene radiation. The face plate 12 is of a suitable material such as glass in the case of a visible light input. A coating 14 of a suitable photoemissive material sensitive to the input radiation, (such as cesium antimony for a visible light input) is provided on the inner surface of the face plate 12. An electron gun 20 is provided at the opposite end of the envelope 10 for generating and forming a pencil-like electron beam which is directed onto a target member 30. The target member 30 is positioned between the electron gun 20 and the photocathode 14. Between the target member 30 and the photocathode 14, there are provided a plurality of electrodes illustrated as 16 and 18 with suitable potentials provided thereon for accelerating and focusing of the photoelectrons emitted from the photocathode 14 onto the target member 30. Positioned between the target member 30 and the electron gun 20, there is provided a grid member 40 of an electrically conductive material such as nickel which is located at a distance of about 0.125 inch from the surface of the target member 30.

The target member 30 is comprised of a support ring 32 of a suitable material such as Kovar alloy (Westinghouse Electric Corporation trademark for an alloy of nickel, iron and cobalt) having a suitable electrically conductive support film 34 such as aluminum attached to the metal ring 32. A porous coating or film 36 of a suitable insulating or semiconducting material which exhibits the property of generation of electrons in response to electron bombardment of one surface which are emitted as secondary from the opposite surface and permitting conduction of the electrons through the voids of the coating, is provided on the conductive back plate 34 facing the electron gun 20. The coating 36 may be of any suitable material such as an alkaline or alkaline earth metal compound, such as potassium chloride, magnesium chloride or magnesium oxide. A mesh or screen 40 serves as a collector for the secondary electrons emitted from the exit or exposed surface of the coating 36. The mesh 40 also contributes in maintaining a uniform electric field between the collector 40 and the target 30. In addition, a conductive coating 44 is provided on the inner wall of the envelope 10 in the space between the electron gun 20 and the target 30 at the potential of screen 40 for providing a suitable electrostatic field.

The electron gun 20 is of any suitable type for producing a low velocity pencil-like electron beam to be scanned over the surface of the target electrode 30. The electron gun 20 may consist of a cathode 22, a control grid 24 and an accelerating grid 26. The gun electrodes 22, 24, and 26 along with the coating 44 provide a focused electron beam which is directed onto the target member 36. Deflection means 50 illustrated as a coil is provided around the envelope 10 for deflection of the electron beam and by application of suitable potentials scans the beam over the surface of the target 30 in a conventional manner. A magnetic coil 52 is also provided around the envelope 10 to provide additional focusing of the electron beam from the read gun 20 onto the target 30 as well as for focusing the electrons from the photocathode 14 onto the target 30.

A specific example of a suitable storage target 30, will now be described. The aluminum film 34 may be formed y the vacuum deposition of aluminum onto a film of thermally removable organic material such as cellulose nitrate. The thickness of the aluminum layer 34 should be about 1,000 angstroms for an electrode of a diameter of about one inch. The cellulose nitrate is baked outleaf/mg the aluminum film 34 attached to the ring 32".- This technique is well known and is described in US.- Patent assigned to the same assignee.

In the specific device described here the film 34. provides the support. A support layer may also be of the type described in US. Patent 2,898,499 assigned to the same assignee as the present invention, The support member must have sufiicient conductivity means so as to replenish electrons.

The film 34 and support ring 32 are then placed in a bell jar having an atmosphere of approximately 1 millimeter mercury of argon or any other inert gas. A boat of suitable material such as tantalum provided with a resistive heat element is positioned with the bell jar. A predetermined amount such as 16 milligrams of a suitable material such as potassium chloride is placed in the boat. The boat is then placed at a distance of approximately three inches below the aluminum layer 34 and current is applied to the resistive heating element of the boat. The heat is applied until the material has just melted at which temperature the material is then maintained. This temperature is considerably less than the melting point of the material at atmospheric pressure. The vapor pressure of the material at its melting point under such conditions is found suflicient to cause vaporization of the material at a sufiicient rate. The material is evaporated to completion and it is found that density of the evaporated storage material on the aluminum layer is approximately 1 to 5 percent of its bulk density. The layer or film 36 has a thickness of approximately 20 microns.

The values of representative potentials applied to the electrodes are illustrated in FIG. 1. The photocathode 14 is operated at a potential of about 8,000 volts negative with respect to the conductive back plate 34 to provide acceleration of the electrons from the photocathode 14, emitted in response to radiations from a scene 51 directed onto the photocathode 14, to the target 30. The back plate 34 of the target 30 may be operated at about 5 volts positive with respect to the gun cathode 22. The surface of the porous storage member 36 is stabilized or maintained at an equilibrium potential which may be substantially ground potential by means of the scanning electron beam from the gun 20. The cathode 22 of the electron gun 20 is connected to ground potential. The electrodes 40 and 44 are maintained at approximately a potential of 250 volts positive with respect to ground and accelerate the electron beam. The retarding field existing between the target 30 and the electrode 40 reduces the velocity of the electron beam so that it approaches the target 30 at a low potential below the first crossover potential of the layer 36. Electrons are deposited on the exposed surface of layer 36 so that surface will seek an equilibrium potential which is substantially equal to the potential of the cathode 22 of the electron gun 20 which is at ground potential.

The radiations from the scene 51 are focused on the photocathode 14 and photoelectrons are emitted from each portion of the photocathode 14 corresponding to the amount of light directed thereon. The photoelectrons are focused upon the target member 30. The photoelectrons are accelerated to a sutficiently high energy of about 8,000 electron volts so that they penetrate through the conductive layer 34 and enter into the layer 36. The acceleration voltage should be adjusted such that substantially all of the primary electrons from the photocathode 14 almost completely penetrate the entire storage electrode 30 but do not substantially pass on through the structure, For example, in the case of a target having a conductive layer 34 of aluminum of a thickness of about 1,000 angstroms and a porous insulator of a thickness of about 20 microns the acceleration voltage would be about 8,000 volts. The primary electrons on passing through the conductive layer 34 of aluminum of about 1,000 angstroms in thickness lose about 25 percent of their initial energy and the remaining 75 percent having about 6,000 electron volts energy are dissipated within the low density layer 36 of transmission secondary emission material. The primary electrons from the photocathode 14 create a certain number of low energy electrons within the layer 36, orders of magnitude higher than the number of incident or primary electrons. The number of secondary electrons generated may be about 200 per each primary. If the ionization energy is about 30 electron volts, then roughly 200 free electrons may be generated within the layer. If the target 30 has been polarized prior to the impact of the signal or writing electrons, which is done by applying a positive potential of about 5 volts to the back plate 34 and stabilizing the exposed surface or exit surface of the target at ground potentials, the low energy electrons generated in the layer 36 cause the exit surface to change its potential locally due to conduction of the electrons across the layer 36 through the vacuum space or voids between the particles of the porous layer to the positive back plate 34 and due to secondary emission transmitted from the exposed surface of layer 36 which are collected by electrode 40. Such a local change of exit surface potential can be employed of course to generate a video signal using any of the several well known readout techniques. In FIG. 1, there is illustrated a typical vidicontype readout assembly.

In order to more fully explain the operation of the target 30, the exit surface potential (V of the target 30 as a function of time for a given input current density I is described. If it is first assumed that the exit surface potential V is at ground or at the gun cathode potential at a time T :0, bombardment of the target 30 by electrons from the photocathode 14 (the writing gun) produces secondary emission from the exit surface of layer 36 and conduction of electrons through the voids of the layer 36. The surface potential V drifts toward the potential (V of the back plate 34 due to the field applied across said layer 36. While, at first, the effect of conductivity of the electrons within the voids of the layer 36 dominates and the transmitted secondary emission contributes only a small fraction of the total change in charge, the situation reverses when the exit surface potential V approaches back plate V This is clearly illustrated in FIG. 5. The conduction due to electrons Within the voids of the target becomes zero when the exit surface V is at the same potential as the back plate 34 as illustrated in FIG. 5. The secondary emission, however, continues and drives V past target back plate potential V which results in a reversal of polarity of electric field across the layer 36. If V is a few volts positive with respect to the potential V of the back plate, then conduction of electrons again takes place through the voids, however, in a reverse direction and at a lesser slope, as indicated in FIG. 5. It is found that by utilizing a layer 36, which exhibits field enhanced secondary electron emission, the secondary emission gain will be greater than the now negative gain due to electron conduction Within the voids of the layer 36 until the surface is stabilized at a certain potential V The potential V is determined by the voltage at which the number of secondary electrons leaving the surface equals the number of free electrons collected by the back plate 34. This potential is illustrated as V in FIG. 5. FIG. 6 illustrates the integrated signal as a function of the exit surface potential.

From the preceding, it is obvious that the following minimum conditions have to be met in order to operate a target 30 in the described manner. The target 30 must exhibit secondary emission from the surface and also provide conduction of free electrons within the voids of the porous layer 36. The capacity of the target 30 should be low enough to achieve lagless discharge. The thickness of the porous layer 36 provides a capacitive advantage by providing a thicker target and yet providing an area density low enough to employ moderate acceleration voltages. The layer 36 must also have a very high electrical resistivity and precautions must be taken to avoid runaway of the exit surface. This runaway effect will be described with respect to FIG. 3. The resulting device provides a high gain of 200 to 300 within the target at accelerating voltages of about 8 kilovolts for the primary electrons. The target 30 also provides a fast response or lagless discharge so as to provide complete removal of the image within less than of a second which is the standard frame time. The target also pro vides an extremely long integration time, greater than sixty minutes, due to the high resistivity of the target layer 36 which is greater than 10" ohm-cm. It is also important to note that an image can be stored on the target 30 for hours with the scan beam turned off". The target 30 is lagless in that no after image is found due to trapped electrons as is found in the conventional electron bombardment induced conductivity type of target. This is due to the fact that the field impressed across the layer 36 is limited to less than 10 volts per centimeter so that the back plate 34 collects electrons conducted through the voids in the layer 36. Conventional electron bombarded induced conductivity due to flow of carriers through the particles of the material in the layer 36 does not contribute, in that, the low field of about 2.5 to kilovolts per centimeter is unable to conduct the carrier through the particles of the layer 36. The mobility of the electrons in the voids is believed to be about 300 cm. /volt sec. The effect of the field is illustrated in FIG. 4. The target 30 also provides adequate resolution of greater than 20 line pairs per millimeter. The well known defects normally found in electron bombardment induced conductivity type targets due to the high field necessary in these devices are not found even after prolonged operation. In addition, the structure utilizing potassium choride as the material in layer 36 may be baked to 325 C., without danger of destruction of the target.

Now referring to FIG. 3, there is illustrated a modified electrode assembly in which an auxiliary grid 41 is positioned between the target member 30 and the conventional collector grid 40 utilized in the vidicon scan read out. In this embodiment, as illustrated in FIG. 3, the collector grid 40 is operated at about 450 volts positive with respect to the cathode and positioned at a distance of about 0.125 inch from the target member 30. The auxiliary grid 41 is positioned between the target 30 and the collector grid 40 and is operated at a potential of about 50 volts positive with respect to ground. The grid 41 is positioned at about 0.0622 inch from the target member 30. As indicated in the previous discussion, under certain conditions the exit surface potential of the target 30 will charge in a positive direction due to the transmission secondary emission and conduction electrons. When the exit surface of layer 36 reaches the potential of the back plate V then continued charging of the exit surface will continue in the positive direction due to domination of transmission secondary emission over the free electron conduction current. It is possible with some material that the exit surface will charge positive with respect to the cathode of the scanning electron beam to a point where the acceleration of the incident reading beam of electrons onto the exposed surface of the target 30 will exceed the first crossover potential of the material in layer 36. This expression is well known in the art. A characteristic of secondary emissive materials is the fact that at low primary energies the number of reflected sec ondary electrons from a surface is less than the number of primaries and therefore the surface will tend to charge in a negative direction. At some point, the energy of the primary electron beam will be such as to create more reflected secondaries from the surface than primaries incident thereon. This results in the surface tending to charge in a positive direction. The potential at which the number of secondary electrons equal the number of primaries is called first crossover potential. If the exposed surface of layer 36 exceeds first crossover potential, then not only would the exposed surface be charging positively due to transmission secondary emission from bombardment of target by the writing beam but also the scanning beam will tend to charge the exit surface positive. This would result in the voltage on exit surface of layer 36 building up to such a value so as to destroy the insulator or semiconductor layer 36 by impressing a strong field thereacross and causing breakdown. By inserting the auxiliary grid 41 between the collector grid 40 and the target 30 and maintaining it at a positive potential below the first crossover or breakdown potential, then one may limit the exit surface potential to prevent run away and possible destruction of the porous layer 36. Since the exit surface cannot exceed the potential of this auxiliary grid 41, the target 30 will not go above first crossover and break down regardless of the intensity of the input signal or integration time. The mode of target operation with respect to the writing and reading processes will not be adversely affected, if the voltages and distances of the electrodes are properly chosen in order to avoid moire pattern, which is well known to those skilled in the art.

In order to more fully define the invention herein, the curve illustrated in FIG. 4 is referred to. In FIG. 4, the gain of the target 30 is plotted with regard to the field across the porous layer 36. As indicated with no field impressed across the layer 36, the gain of the target 30 is due entirely to the emission of secondary electrons from the exposed or exit surface of the porous body 36. As the field is applied across the porous layer with the back plate 34 positive with respect to the exit surface, the gain of the target 30 increases due to conduction of free electrons through voids of layer 36 to a point roughly equal to a gain of two hundred. Beyond this point, it should be noted that the slope of the gain of the target is less and this gain is due to electrons within the conduction band due to conventional electron bombardment induced conductivity phenomenon wherein charge carries more through the solid material. In this range, the field across the porous layer 36 is high enough to allow the electrons to penetrate inter particle barriers and provide a solid state charge flow. The conduction current within the low field region provides a target of fast response about 5 of a second or less while with a high field and conventional electron bombardment induced conductivity the target is slow about /2 of a second or more.

While there have been shown and described what are presently considered to be the preferred embodiments of the invention, modifications thereto will readily occur to those skilled in the art. For example in the Writing section, a scanning electron beam system may be substituted for the photocathode 14 to provide a scan converter type of tube.

In addition, the storage system described herein may be used in a transmission type of storage tube such as described in U.S. Patent 3,002,124 and assigned to the same assignee. In the transmission storage tube application, the porous storage layer could be operated so that the charging due to conduction of electrons within the porous layer would be enhanced by reflected secondary electrons. Both the write and read electron guns would be disposed on the exposed surface side of the storage electrode which would be in the form of an apertured electrode. In addition to writing with electrons, light or similar radiations may be directed onto a suitable porous storage layer such as KCl to produce electrons by photoemission inherent in the material or b yaddition of suitable photoemissive material such as cesium antimony.

It is not desired, therefore, that the invention be limited to the specific arrangements shown and described, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

We claim as our invention:

1. An electron discharge device comprising a storage electrode including an electrically conductive means, a porous film deposit of from one to five percent of its normal bulk density of high resistive material deposited on one surface of said conductive means and having the property of generating electrons in response to electron bombardment, means for directing a writing electron beam modulated With a signal at said target of a predetermined energy to penetrate said porous film to generate secondary electrons within said porous film, means for establishing a field across said porous film to collect said secondary electrons emitted into the vacuum spaces between particles but inadequate to collect charge carriers within the solid material, and means for directing electrons below said predetermined energy at said exposed surface of said porous film to restore said exposed surface to an equilibrium potential while deriving a signal corresponding to the signal on said writing beam.

2. An electron discharge device comprising a target electrode, said target electrode including a porous layer of less of than 10 percent of the normal bulk density of a high resistive material exhibiting the property of emitting secondary electrons from first surface thereof in response to electron bombardment of a second surface, an electrically conductive coating provided on said second sur face, means directing a writing electron. beam having electrons of a predetermined energy at said conductive coating to penetrate said coating and generate secondary electrons within said porous layer such that a portion of the secondary electrons are emitted from the exposed surface of said porous layer, means establishing a field across said target such that the conductive backing is initially at a positive potential with respect to the potential on said first surface to impress a field across said porous layer adequate to conduct electrons through to the voids in said porous layer but inadequate to conduct charge carriers through the particles in said layer, so that secondary electrons generated Within said porous layer and not emitted from said first surface are collected by said conductive member to enhance the positive charge pattern established on said first surface from an equilibrium charge corresponding to the intelligence in said Writing beam, and means directing electrons below said predetermined energy at said first surface of said porous layer to restore said first surface to said equilibrium potential.

3. An electron discharge device comprising a target electrode, said target electrode including a porous layer of less than 10 percent of the normal bulk density of potassium chloride of a thickness of about 20 microns, a layer of conductive material positioned on one surface of said porous layer, means for directing a writing electron beam onto said target through said conductive layer to create electrons within said porous layer of which a first portion is emitted from said porous layer as secondary electrons to create a positive charge at the exposed surface of said porous layer, means establishing an equilibrium potential on the exposed surface of said porous layer substantially corresponding to the cathode of a reading electron beam, the equilibrium potential one said exposed surface of said porous layer being at a negative potential with respect to a potential applied to said conductive back plate to establish a field of less than 10 volts per centimeter across said porous layer to collect a second portion of said electrons generated within said porous layer in response to electron impingement by said primary beam to enhance the positive charge produced at the exposed surface of said porous layer.

4. An electron discharge device comprising a target electrode said target comprised of an electrically conductive layer, a porous film of less than 10 percent of the normal bulk density of an insulating material deposited on said conductive layer exhibiting the property of emission of secondary electrons in response to electron bombardment and generation of electrons Within said porous film, means impressing a field of less than 10 volts per centimeter without creating solid state conduction across said porous film, means directing a Writing electron beam at said target having electrons of predetermined energy to penetrate said conductive layer and enter said porous 10 film and generate free electrons, a first portion of which are emitted and a second portion collected by said conductive layer to establish a positive charge pattern from said equilibrium charge corresponding to said writing beam and means for directing electrons below said predetermined energy at the exposed surface of said porous film to restore said first surface to said equilibrium charge.

5. An electron discharge device comprising a target electrode, said target electrode including a porous layer of less than 10 percent of the normal bulk density of a high resistive material exhibiting the property of emitting secondary electrons from first surface thereof in response to electron bombardment of a second surface and conduction of electrons generated by bombardment in response to a low energy field without solid state conduction to provide fast response, a conductive coating provided on said second surface, means directing a writing electron beam having electrons of a predetermined energy at said conductive coating to penetrate said coating and generate secondary electrons Within said porous layer such that said first portion of the secondary electrons are emitted, means establishing a field across said target such that the conductive backing is at a positive potential with respect to the potential on said first surface so that said second portion of secondary electrons generated within said porous layer are collected by said conductive member to enhance the positive charge pattern established on said first surface from an equilibrium charge corresponding to the intelligence in said writing beam, said field inadequate to collect charge carriers within the solid material of said porous layer, and means for directing electrons below said predetermined energy at said first surface of said porous layer to restore said first surface to said equilibrium potential.

6. An electron discharge device comprising a target electrode, said target electrode including a porous layer of less than 10 percent of the normal bulk density of an insulating material of a thickness of about 20 microns, a layer of electrically conductive material positioned on one surface of said porous layer, means directing a writing electron beam onto said target through said conductive layer to create free electrons Within said porous layer of which a first portion are emitted from said porous layer as secondary electrons and thereby creating a positive charge at the exposed surface of said porous layer, means establishing an equilibrium potential on the exposed surface of said target substantially corresponding to the cathode of an electron beam, the equilibrium potential on said exposed surface of said porous layer being at a negative potential With respect to the potential applied to said conductive back plate and thereby establish a field of about 2.5 to 5 kilovolts per centimeter across said porous layer and collect a second portion of said conduction electrons generated within said porous layer in response to electron impingement by said primary beam to enhance the positive charge produced at the exposed surface of said porous layer without resort to solid state conduction.

7. An electron discharge device comprising a storage electrode including an electrically conductive means, a porous film of less than 10 percent of the normal bulk density of an insulating material deposited on said conductive means and having the property of generating electrons in response to an input radiation, means for directing said input radiation onto said porous film to cause generation of electrons within said porous film, means establishing a field across said porous film adequate to collect said electrons generated in said porous film but inadequate to collect charge carriers Within said solid material of said porous film, and thereby establish a positive potential on the exposed surface of the porous film, and means for directing electrons at said exposed surface of said porous film to restore said exposed surface to an equilibrium potential while simultaneously deriving a signal corresponding to said radiation input.

8. An electron discharge device comprising a target electrode, said target electrode including a porous layer of less than percent of the normal bulk density of an insulating material exhibiting the property of emitting secondary electrons from a first surface in response to electron bombardment of said layer, an electrically conductive coating provided on the second surface of said porous layer, means directing a writing electron beam having electrons of a predetermined energy at said target electrode to penetrate said porous layer and produce secondary electrons Within said porous layer such that a portion of the secondary electrons are emitted from the first surface of said porous layer, means establishing a field across said porous layer such that the conductive backing is at a positive potential with respect to an equilibrium potential on said first surface so that secondary electrons produced within said porous layer and not emitted from said first surface are collected by said conduc tive member to enhance the positive charge pattern established on said first surface from an equilibrium charge corresponding to the intelligence in said writing beam, said field inadequate to collect charge carriers within the solid material of said porous layer, and means for directing electrons below said predetermined energy at said first surface of said porous layer to restore said first surface to said equilibrium potential.

9. An electron discharge device comprising a target electrode, said target electrode including a porous layer of less than 10 percent of the normal bulk density of an insulating material of a thickness of about microns, a layer of conductive material positioned on one surface of said porous layer, means directing a writing electron beam onto said target to create free electrons within said porous layer of which a first portion is emitted from said porous layer as secondary electrons and thereby tending to charge the exposed surface of said porous layer, toward a potential applied to said conductive layer, means establishing an equilibrium potential on the exposed surface of said porous layer by means of a reading electron beam, the equilibrium potential on said exposed surface of said porous layer being at a different potential with respect to the potential applied to said conductive layer and thereby establish a field of about 2.5 kilovolts per centimeter across said layer to collect a second portion of said free electrons generated within said porous layer in response to electron impingement by said primary beam to enhance the charge produced at the exposed surface of said porous layer.

10. An electron discharge device comprising a target electrode and electrically conductive means, a film of insulating material in the form of a porous deposit of less than 10 percent of its normal bulk density on said conductive means exhibiting the property of emission of secondary electron from one surface in response to electron bombardment of the other surface and production of free electrons within said film and capable of being collected by a low energy field without creating solid state conduction to provide fast response, means directing a writing electron beam at said target having electrons of predetermined energy to enter said film and generate free electrons, a first portion of which are emitted from the exposed surface and a second portion collected by said conductive layer in response to said low energy field across said film to establish a positive charge pattern from an equilibrium charge corresponding to said writing beam and means for directing electron of less energy than said predetermined energy at said first surface of said film to restore said first surface to said equilibrium charge and derive a signal corresponding to said writing beam.

11. An electron discharge device comprising a target electrode, said target electrode including a porous layer of less than 10 percent of the normal bulk density of a high resistive storage material exhibiting the property of emitting secondary electrons from first surface thereof in response to electron bombardment of a second surface and conduction of electrons generated by bombardment in response to a low energy field without solid state conduction to provide fast response, a conductive coating provided on said second surface, means directing a writing electron beam having electrons of a predetermined energy at said conductive coating to penetrate said coating and generate secondary electrons within said porous layer such that a first portion of the secondary electrons are emitted, means establishing a field across said porous layer such that the conductive backing is at a positive potential with respect to the potential on said first surface so that a second portion of secondary electrons generated within said porous layer are collected by said conductive member to enhance the positive charge pattern established on said first surface from an equilibrium charge corresponding to the intelligence in said writing beam, said field inadequate to collect charge carriers in the solid material of said porous layer, and means for directing electrons below said'predetermined energy at said first surface of'said film porous layer to restore said first surface to said equilibrium potential.

12. An electron discharge device comprising a target electrode, said target electrode including a porous layer of less than 10 percent of the normal bulk density of a storage material of a thickness of about 20 microns, a layer of electrically conductive material positioned on one surface of said porous layer, means directing a writing electron beam onto said target through said conductive layer to create free electrons within said porous layer of which a first portion is emitted from said porous layer as secondary electrons and thereby creating a positive charge at the exposed surface of said porous layer, means establishing an equilibrium potential on the exposed surface of said target substantially corresponding to the cathode of reading electron beam, the equilibrium potential on said exposed surface of said porous layer being at a negative potential with respect to the potential applied to said conductive back plate and thereby establish a field of about 2.5 kilovolts per centimeter across said porous layer and collect a second portion of said conduction electrons generated within said porous layer in response to electron impingement by said primary beam to enhance the positive charge produced at the exposed surface of said porous layer without resort to solid state conduction.

References Cited by the Examiner UNITED STATES PATENTS 2,544,753 3/51 Graham 313 2,960,617 11/60 Lodge 315-10 3,048,502 8/62 Nicholson 3 1365 .1

DAVID G. REDINBAUGH, Primary Examiner. 

1. AN ELECTRON DISCHARGE DEVICE COMPRISING A STORAGE ELECTRODE INCLUDING AN ELECTRICALLY CONDUCTIVE MEANS, A POROUS FILM DEPOSIT OF FROM ONE TO FIVE PERCENT OF ITS NORMAL BULK DENSITY OF HIGH RESISTIVE MATERIAL DEPOSITED ON ONE SURFACE OF SAID CONDUCTIVE MEANS AND HAVING THE PROPERTY OF GENERATING ELECTRONS IN RESPONSE TO ELECTRON BOMBARDMENT, MEANS FOR DIRECTING A WRITING ELECTRON BEAM MODULATED WITH A SIGNAL AT SAID TARGET OF A PREDETERMINED ENERGY TO PENETRATE SAID POROUS FILM TO GENERATE SECONDARY ELECTRONS WITHIN SAID POROUS FILM, MEANS FOR ESTABLISHING A FIELD ACROSS SAID POROUS FILM TO COLLECT SAID SECONDARY ELECTRONS EMITTED INTO THE VACUUM SPACES BETWEEN PARTICLES BUT INADEQUATE TO COLLECT CHARGE CARRIERS WITHIN THE SOLID MATERIAL, AND MEANS FOR DIRECTING ELECTRONS BELOW SAID PREDERMINED ENERGY AT SAID EXPOSED SURFACE OF SAID POROUS FILM TO RESTORE SAID EXPOSED SURFACE TO AN EQUILIBRIUM POTENTIAL WHILE DERIVING A SIGNAL CORRESPONDING TO THE SIGNAL ON SAID WRITING BEAM. 